Suitable solid state hydrogen storage materials to be used in the development of onboard hydrogen-fueled vehicles is of great interest. Among the most promising candidate materials is alane (AlH3) due to its high hydrogen capacity (10.1 wt %) and high volumetric of hydrogen of 149 kg/m3, which is twice that of liquid hydrogen. Although alane is unstable thermodynamically, when in crystalline form it is kinetically metastable at room temperature. The alpha phase of alane (α-AlH3) has a Gibbs free energy of formation (ΔG°f) of 48.5 kJ/mol-AlH3 at 298 K which would make it more likely to exist as elemental Al and H2 according to Eq. (1); however, α-AlH3 exists quite readily at room temperature. Consequently, alane can be easily handled and controlled to desorb hydrogen at desirable rates and sufficient temperatures required for optimal fuel cell operation. Beside its potential use in hydrogen-powered cars, alane can also be useful in a wide array of other applications such as reagents in chemical reactions, rocket fuel, and portable power system.Al+3/2H2→α-AlH3ΔG°f[α-AlH3]=48.5 kL/mol  (1)
The most direct route to synthesize alane is hydrogenation of aluminum shown in Eq. (1). This method is difficult due to the high ΔG°f and would require a H2 pressure greater than 0.7 GPa, the equivalent equilibrium pressure of reaction (1) at room temperature. Literature reviews indicate AlH3 can be generated when aluminum is pressurized with 2.5 GPa hydrogen pressure. It is also possible to generate limited AlH3 crystals from pulverized aluminum and 8.9 GPa H2, at 600° C. It is also possible to generate alane by hydrogenating aluminum at a relatively low pressure in presence of titanium nanoparticles and triethyldiamine (TEDA). However, the difficulty associated with this method is removing TEDA from the AlH3.TEDA adduct to form AlH3.
Presently, industrial quantities of AlH3 are commonly synthesized in solution by a chemical metathesis reaction between AlCl3 and LiAlH4. The reaction proceeds very favorably as shown in Eq. (2). Because AlH3 is thermodynamically unstable at room temperature, the reaction is typically carried out in donating solvents such as ethers or amines in order to stabilize monomeric alane as a solvate AlH3·nL (L=Et2O, amines or THF) as depicted Eq. (3). The solvent is subsequently removed via heating and vacuum pump yielding a stabilized polymeric form of alane (AlH3)n. In an industrial scale, the removal of solvent adducts by vacuum pump represents a significant energy cost in terms of the electricity being consumed. Moreover, the handling of bulk quantities of the pyrophoric AlH3 etherate adduct and ether solvents is a definite hazard.
Therefore, it is more desirable to have a solid state synthetic route to reaction (2) in which solvents are avoided or limited for use only during workup.AlCl3+3LiAlH4→4AlH3+3LiCl ΔrG298K=−191 kJ/mol  (2)AlCl330 3LiAlH4+n[(C2H5)2O]→4AlH3*1.2[(C2H5)2O]+3LiCl  (3)
Heretofore, direct routes to synthesize alane are not commercially feasible given pressure requirements needed to carry out a reaction at ambient temperatures. Further, current synthesis techniques result in an alane adduct such as alane-etherat adduct. Removal of the ether is an impediment to use of the synthesis techniques in the prior art. Accordingly, there is room for improvement in the art.